In a typical cellular network, also referred to as a wireless communication system, User Equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks (CNs).
A user equipment is a mobile terminal by which a subscriber can access services offered by an operator's core network. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, laptops or tablet computers, sometimes referred to as surf plates, with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wirelessly in the cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The cellular network covers a geographical area which is divided into cell areas. Each cell area is served by a Base Station, BS, or Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used.
The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pica base station, based on transmission power and thereby also on cell size.
A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to one or more core networks.
UMTS is a third generation (3G) mobile communication system, which evolved from the second generation (2G) mobile communication system (GSM) and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
In the context of this disclosure, a base station or radio base station as described above will be referred to as a Base Station (BS). A user equipment as described above, will in this disclosure be referred to as a User Equipment or a UE.
The expression Downlink (DL) will be used for the transmission path from the base station to the user equipment. The expression UpLink (UL) will be used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
Cellular communication networks evolve towards higher data rates, together with improved capacity and coverage. In 3GPP, standardization body technologies like GSM, HSPA and LTE have been and are currently developed.
To provide for mobility in the cellular network, the cellular network must perform so called handovers when the user equipment moves from one cell to another. A handover means that there is a change of serving cell for the user equipment from a so-called source cell to a so-called target cell. There are mechanisms in the cellular network to identify which cells are candidate target cells for handover. Typically, the user equipment regularly performs measurements to monitor which cells provide coverage at its current location. The measurement result is sent to a serving base station of the source cell in so called measurement reports. These measurement reports may be used to trigger a handover to the target cell in due time before the user equipment moves out of coverage from the source cell.
Apart from handovers, measurements are also made by the UE to enable a base station or control node to decide on the length of the Transmission Time Interval (TTI) to be deployed. The length of the TTI influences the performance of the radio communication between UE and BS. A relatively long TTI length has the advantages of being efficient in error correction and consuming relatively low power, while a relatively short TTI length has the advantages of providing higher data rates and enabling faster adaptation to a changing radio environment.
When a UE is close to the center of the serving cell, so in general close or in short range to the serving BS antenna, a relatively short TTI is applied. When however the UE is moving away from the center of the served cell, the disadvantage occurs that due to the distance the power required to maintain the BS-UE radio link has to be increased. Increasing power has a disadvantageous effect to other UEs in the same cell and should be limited in order to maintain the radio links between the other UEs and the BS. When reaching this limit, an alternative can be applied to maintain said BS-UE radio link in deploying a lower power scheme by switching to a longer TTI.
The actual deployment of either the shorter or longer TTI length is a tradeoff between a.o. the parameters listed above
For supporting mobile broadband services, good latency is essential in providing good end-user experience. This requires short round-trip time enabled by relatively short TTI.
For the Enhanced UpLink, EUL, the value used for a short TTI is therefore preferred over the long TTI. In current 3G networks, there is still a substantial amount of large macro cells where supporting the short TTI in the entire cell may be a challenge. In such environments, it may be necessary to apply to the long TTI for a UE residing at the cell boundary.
A problem is that, the BS and the UE should switch to a different TTI, listed as a TTI switching process, at the same instant, and it is regarded that this is a process of that takes time and takes power, e.g power from the battery fed UE, causing the TTI switching process reducing available lifetime of the operational up-time of the UE.
Another problem is that the TTI switching process requires signaling that is applied via the radio environment, thereby competing with, or at least influencing or interfering, other UEs for radio links with the BS, and can be disadvantageously influenced by changing radio environment conditions, which may result in an unsatisfactory HI switching process. Failed transmissions of data after an unsatifying TTI switch may occur.